Turbine rotors, for example those used in steam turbines, have an essentially cylindrical base unit, on which several disc-shaped cylindrical sections or rings are formed. These are provided on their periphery with spaced grooves to receive the turbine blades, the flanks of which are toothed in the longitudinal direction of the grooves. The grooves therefore have a branched, tapering, stepped profile (see FIGS. 1A and 8A), hereinafter called "Christmas tree" profile, which serves to positively receive and secure a foot of a turbine blade. In this case, straight grooves running in the longitudinal direction as well as oblique grooves and grooves in the shape of a circular arc are known.
A process and device for forming rotor grooves in the shape of circular arcs when viewed from the side are known from the article "Methoden und Maschinen zur Frasbearbeitung von Generator-und Turbinenrotoren" Methods and machines for milling generator and turbine rotors!, Ing. (Grad.) Siegfried Neumann, special publication No. 9 of Kollmann Maschinenbau GmbH, 5602 Langenberg/Rhld. The circular arc-shaped grooves are constructed from a blank with disc-shaped cylinder sections. A milling machine is used to manufacture the grooves. The machine has a tool spindle which is mounted to be axially adjustable. The axial direction of the tool spindle essentially corresponds to a radial direction of the tool or is parallel thereto. One special bell-shaped cutter, termed a "bell cutter" or "shell milling cutter" is attached to the tool spindle; its tool base unit has the shape of a circular plate with an axially projecting edge. The contour of the edge, viewed in longitudinal section, has cutting elements bearing cutting edges. The circumferential outline of the cutting edges determines the contour of the groove to be cut in a rough cutting process and initially coarsely pre-shaped. The profile is composed of several rectangles and tapers towards the edge of the bell cutter in stages. It is free from undercuts and serves merely to open a roughly cut pre-contoured groove.
A circular milling machine is used to shape and finish the groove. The milling machine moves and guides a shank-type profile cutter in forward feed on a circular arcuate path along the groove. The shank-type cutter has a profile which corresponds to the shape of the groove flank to be produced. The shank-type profile cutter has undercuts relative to the radial direction of the work piece, e.g. the turbine rotor.
Because of the large profile depth required for construction of the undercuts and the long cutting path corresponding to the groove length, milling with the shank-type cutter requires a long manufacturing. With a larger number of grooves distributed along the periphery of the turbine rotor, this adds to a considerable total operating time which, in an example can amount to appreciably more than 5 hours for the grooves on one circular periphery alone. The long machine time required results in high production costs in the manufacture of turbine rotors.